1. Field of the Invention
The present invention pertains to a driving apparatus that uses as a drive source a member that expands and contracts, and more particularly, to a driving apparatus that employs an electromechanical transducer such as a piezoelectric element, e.g., to a driving apparatus used for the precision driving of an X-Y drive table, a camera image recording lens, or a probe of a scanning tunneling electron microscope.
2. Description of the Related Art
Conventional examples of a driving apparatus using a piezoelectric element include driving apparatus 10a in which movable unit 20a moves along shaft 14, as shown in FIGS. 1 through 3, and driving apparatus 10b in which movable unit 20b moves along guide groove 18, as shown in FIGS. 4 through 6. In these driving apparatuses 10a and 10b, an elastic deforming mechanism is constructed by pressing protrusion 24a or 25a of plate spring 24 or 25, which is a friction member connected to one end of piezoelectric element 22, against the outer surface of shaft 14 or against the inner surface of guide groove 18, such that a frictional force may be created. A driving apparatus of the type in which the friction member has an elastic deforming mechanism which creates frictional force is superior to a driving apparatus of the type in which a frictional force is created by applying external force to the friction member by means of a spring member, as shown in U.S. Pat. No. 5,589,723, in that the construction may be simplified.
In either driving apparatus 10a or 10b, the elastic deformation stress vector that occurs during the elastic deformation of protrusion 24a or 25a of plate spring 24 or 25 runs in the directions indicated by bi-directional arrow 82 or 84 in FIG. 3 or 6, where stress is applied that has a component that works in the directions of expansion and contraction of piezoelectric element 22, as shown by bi-directional arrow 80. In other words, deflection occurs in plate spring 24 or 25, the friction member, in the directions of expansion and contraction of piezoelectric element 22, between the area that is fixed to piezoelectric element 22 and the area that is in frictional contact with shaft 14 or guide groove 18, and plate spring 24 or 25 elastically deforms in expansion and contraction directions 80 of piezoelectric element 22.
Therefore, if the frequency of the pulse voltage that is applied to piezoelectric element 22 is increased in order to move movable unit 20a or 20b of driving apparatus 10a or 10b at a high rate of speed, the change in position that occurs at one end 22a of piezoelectric element 22 can no longer be communicated to the frictional contact point via plate spring 24 or 25 in the same manner as before. FIGS. 7 and 8 are graphs that show the transfer function G=Y/X, i.e., the relationship between the transfer of the change in position X of piezoelectric element 22 and the change in position Y of the frictional contact point between friction member 24 or 25 and shaft 14 or groove 18. In other words, as shown in FIGS. 7 and 8, when the frequency increases, the gain decreases and the phase changes. Therefore, as shown in FIG. 9, with conventional driving apparatuses 10a and 10b, when the frequency increases to a certain level, the drive speed of movable units 20a and 20b decreases, as a result of which the drive speed cannot be increased by increasing the frequency.